There's items you can't buy from the store - mostly some specific promotional items (including the Brain Bucket).

Before they added the "Mann Co. Store Package" (which gives you a normal item after buying $20 worth items in one go from the store), you would've gotten an uncraftable random item by spending $20. And you could get an item not obtainable from the store.
These uncraftable items are very uncommon (like OP said, there's only 23 uncraftable Brain Buckets in existence).
 
Let the guy do his business.

There's a fucking rule that says "No complaining about prices". Next one to whine gets a warning point.
 
Yes, rarity is everything. There are like 600 Houwars out there and only 8 Steaming Mongolians, guess which one's worth more.
Guys... this is someones trade thread, can we stop posting here and make a thread in the Trade chat.
Edit: Dammit Jwso...